Various types of engines may use pistons in a cylinder bore. Each piston may reciprocate within its associated bore as a portion of the piston's outer circumferential surface is guided by the cylinder wall. The piston may include a skirt that is shaped to bear against the cylinder wall (with a hydrodynamic layer therebetween to provide lubrication) as the piston is reciprocated in the cylinder bore. In general, the lower portion of the piston within the piston skirt is substantially hollow while the upper portion of the piston near the piston face is solid. Accordingly, the piston may have non-uniform thermal expansion and non-uniform rigidity.
Stress concentrations caused by the piston's thermal expansion, flexing, and rocking in the bore may cause the piston to “polish” or otherwise scuff the surface of the cylinder wall after repeated reciprocating movements. Also, thermal expansion of the piston material may increase the contact force between the piston and bore, causing high friction that may result in loss of efficiency and possible seizure of the piston in the cylinder bore. If the outer radius of the piston is too small, the outer circumferential surface may not sufficiently bear against the cylinder wall-causing the piston to excessively rock on the piston pin axis or vibrate within the cylinder bore.